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The use of unequal randomisation in clinical trials — An update.Objective To update a 2005 review of the reasons researchers have given for the use of unequal randomisation in randomised controlled trials (RCTs). Main measures Intervention being tested; type of study; number of participants; randomisation ratio; sample size calculation and reason given for using unequal randomisation. Methods Review of trials using unequal randomisation. Databases and sources Cochrane library, Medline and CINAHL. Results A total of 86 trials were identified. Of these 82 trials (95%) recruited patients in favour of the experimental group. Various reasons for the use of unequal randomisation were given including: gaining treatment experience; identification of adverse events; ethical; logistic and enhancing recruitment. No trial reported explicitly used it for cost-effectiveness. Most of the papers (i.e. 47, 55%) did not state why they had used unequal randomisation and only 38 trials (44%) appeared to have taken the unequal randomisation into account in their sample size calculation. Conclusion Most studies did not mention the rationale for unequal allocation, and a significant proportion did not appear to account for it in the sample size calculations. Unlike the previous review economic considerations were not stated as a rationale for its use. A number of trials used it to enhance recruitment, although this has not been tested.